kms: add support for kms atomic api

Submitted by Randy Li (ayaka)

Link to original bug (#796516)

Description

Created attachment 372577
A early version

This patch only add a basic flow for atomic API.
I have not done yet.
Except removing kms sync mechanism there is not much different to legacy API.
I am planning to install some kms properties to gobject like v4l2 does. Also GstVideoAggregator is a important feature that would be introduced, as kms atomic api supports commit multiple plane at the same time. With the helper of that, it is possible to show both video and video subtitle with this plugin.

Patch 372577, "A early version":
0003-WIP-kms-support-atomic-api.patch